The performance of class D amplifier can be improved with the use of a self oscillating variable frequency modulator. The most notable advantage is a lower noise floor. The drawback to this approach is that the frequency spectrum of the modulator spans the entire AM band, because the frequency falls as the power output increases--see FIG. 1. Furthermore, the front end bandpass filters used in tuners effectively convert the frequency modulation of a variable frequency modulator into AM modulation, since the amplitude of the interference varies with frequency (slope detection). All of these issues make the modulator in FIG. 1 unacceptable for use with an AM radio. The modulator as shown in FIG. 1 is unacceptable for audio amplification because the frequency transverses the audio range at high powers, and at clipping the control loop opens and the integrator saturates. An improved modulator based on the one in FIG. 1 which limits the frequency above 100 kHz at full power can be found in my co-pending patent application U.S. Ser. No. 09/342,376, filed Jun. 29, 1999, whose entire disclosure is incorporated by reference.
This invention maintains the benefits of self oscillation, and eliminates AM interference by synchronization to an external clock whose frequency is chosen so that it does not interfere with AM reception. The details of how this frequency is chosen are discussed in my co-pending application (U.S. Ser. No. 09/342,376, filed Jun. 29, 1999).